3 What are the limitations of Rietveld? Phase detection limit of 1-5%Impurity atom determinationNew/unknown crystal structuresDetermination of disorder parameterMultiple parameters can have same effect diffraction patternsEquipment (powder vs. single crystal, x-rays vs. neutrons)Garbage in, garbage out! (noisy data, poor quality sample, poor sample prep)
5 Some common challenges/problems False minimasRefinement diverges (“blows up”)Which goodness of fit to choose? R vs. Chi sq?Preferred orientationLarge amorphous backgroundIncorrect starting crystal structure or lattice parameterPoor quality data!
6 Resources and linksGSAS/EXPGUI:GSAS tutorials:FullProf: https://www.ill.eu/sites/fullprof/PowDLL (file conversion):Lots of info about fitting parameters:People: Lauryn Baranowski Lakshmi Krishna